The present invention relates to a method for suppressing phantom images in a digital copying or printing process comprising a developing step in which:
toner material is transferred to or from a surface of a support medium,
each point of said surface is subject to a sequence of process steps,
each pair consisting of a point of said surface and a process step of said sequence is associated with a pixel of image information to be printed, and
said process steps include at least one step which is influenced in a predetermined way by at least one preceding process step for the same surface point.
One example for a digital or printing process to which the present invention is applicable is the electrophotographic process, in which a latent electrostatic charge pattern is formed by a laser or LED exposure of a photoconductive drum or belt in accordance with a dot like image pattern. The charge pattern is then moved past a developing zone, where it is developed with toner powder, e.g. by a magnetic brush process using a donor drum to which the toner adheres by magnetic attraction. The toner is supplied on the surface of the donor drum which, in this case, forms the above-mentioned support medium. The above-mentioned process steps are the steps in which a given point on the surface of the donor drum reaches the developing zone, and toner is either transferred onto the photoconductor or not, depending on the charge state of the photoconductor, i. e. on the image information of the pixel to be printed. Thus, some portions on the surface of the donor drum will loose their toner layer, whereas other portions of this drum remain coated with toner. During further rotation of the donor drum, new toner is supplied from a reservoir, so that an apparently uniform toner layer is re-established on the surface of the donor drum.
It turns out, however, that the toner layer on the surface of the donor drum is not completely uniform. In those portions, where toner had been transferred to the photoconductor and had been replaced by new toner from the reservoir, the thickness, physical structure and/or composition (e.g. particle size distribution) of the toner layer is slightly different from the conditions of the toner layer in the other portions, where a toner layer had constantly remained on the surface. In case of a two-component developer comprising carrier particles and toner particles, a difference may also be caused by the fact that the developer becomes exhausted, i.e. becomes deprived of toner particles. The next process step in the sequence is reached when the same point on the surface of the donor drum reaches the developing zone once again. Due to the above-mentioned differences in the constitution of the toner layer, the process of toner transfer in this subsequent step will to some extent be influenced by the xe2x80x9chistoryxe2x80x9d of the donor drum. This influence may become visible on the printed image in the form of a phantom image.
This effect and other effects leading to phantom images will be explained in greater detail in the description of the preferred embodiments.
The occurrence and intensity of phantom images will generally depend on various parameters of the developing process, such as the strength of electric and/or magnetic fields in the developing zone, gap width or contact pressure in the developing zone, properties of the toner, surface properties of the support medium and the like. When these parameters are known, the intensity of phantom images is predictable and, to some extent, controllable on the basis of empirical data. An important conventional measure for suppressing phantom images is carefully cleaning of the support medium. However, the very properties of the toner which are normally desirable in the developing process, i.e. small particle size, high pigmentation and good adhesion, make the cleaning process difficult. In addition, too intensive cleaning may limit the lifetime of the components involved.
In black and white printing or copying, phantom images will normally be visible only on solid white or black parts of the image, and it is in most cases possible to suppress these phantom images to a sufficient extent by conventional measures. However, in halftone printing and especially in full color printing, the likelihood of visible phantom images is increased, because phantom images will also be visible in areas with a uniformly shaded or colored background in the form of a slight change in hue or density. Further, it is more difficult in these cases to suppress the phantom images by controlling the parameters of the developing process.
It is therefore an object of the present invention to provide a method for more efficiently suppressing phantom images in digital printing or copying, especially in halftone or full color printing or copying.
According to the present invention, in case of a digital copying or printing process as described in the preamble, this object is achieved by a digital image processing step in which the image information for each pixel to be printed is corrected on the basis of image information for at least the pixel associated with the same surface point and said preceding process step, thereby compensating for the influence of this preceding process step. Thus, according to the general concept of the invention, phantom images are suppressed by digital image processing. The invention then takes advantage of the fact that the occurrence and intensity of phantom images is predictable once the parameters of the process, e.g. developing, transfer and cleaning steps as well as average behavior such as aging behavior of the image registration member, intermediate transfer (when used) and toner and/or developer powder, are known. Since the phantom image is overlaid on the image to be printed in accordance with the image information, it is possible to correct the image information in such a way that the combined effect of the corrected image information and the phantom image gives the desired density of the image or, in case of color printing, of the color component to be printed.